Previously, a hygienic tank truck pump of the self-priming type was used for pumping milk from the stationary milk tank of a farmer into the mobile tank truck of the dairy. When the tank truck is coupled to a trailer tank, viz. a trailer for a truck, the pump in question is often used for pumping milk from the tank of the tank truck into the trailer tank. At the dairy the tank of the tank truck, and the trailer tank are, however, emptied by means of another pump.
Previously either a vacuum pump, viz. a rotor compressor or a rotor vacuum pump, or a conventional liquid ring pump was used as tank truck pump due to the demand for an automatic drawing capacity, said pumps being provided with either eccentrically arranged rotor blades or with a worm/screw arrangement in the liquid ring.
The structure of vacuum pumps provides said pumps with a satisfying self-priming capacity, but the pumping capacity (m.sup.3 /h) is very low with the result that the users face long operation periods and a reduced efficiently of the tank truck. In addition, a vacuum pump must be disassembled when it is to be cleaned.
Conventional liquid ring pumps are seldom used because they must be very large, heavy, and energy-demanding in order to yield the necessary capacity. These pumps are not hygienic either as they cannot be satisfactorily cleaned.